Filler for sensor systems of temperature-sensitive displacement pickups

ABSTRACT

2H-heptafluoropropane (HFC 227) is used as filler in sensor systems of temperature-sensitive displacement pickups.

Mechanical switching elements having an adjustable temperature range,so-called temperature-sensitive displacement pickups, such asthermostats in domestic refrigerators and food freezers, in servicewater heaters heated by gas or electricity, or in hot water heatingsystems consist of a sensor element which is connected to a metalbellows via a capillary tube. The sensor element, capillary tube andmetal bellows--termed "sensor system" below--is [sic] filled with afiller, generally a liquefied gas under pressure, If, for example, thesensor element mounted in the service water heater, and thus also thefiller, is heated, its vapor pressure rises, resulting in an expansionof the metal bellows in the longitudinal direction. This expansion isused via a corresponding known arrangement to actuate a valve or totrigger a switching process directly or via a driven conductor. Byvarying a spring biasing which acts on the metal bellows, it can beachieved that the switching process is triggered at a higher or lowervapor pressure in the sensor system in accordance with a higher or lowertemperature. Depending on the application, that is to say depending onthe temperature range within which the switching temperature is to bevariable, a suitable filler having an appropriate vaporpressure/temperature dependence is required in order that, on the onehand, the design requirements with respect to loadability in compressionand spring biasing can be justified and on the other hand, however, thatthere is a sufficiently large quotient δp/δt to ensure as low aspossible a switching hysteresis between the heating and coolingprocesses.

Sulphur dioxide is known as filler for temperature-sensitivedisplacement pickups. Although the amounts filled per filler system arerelatively small (between approximately 2 and 5 g depending on the typeof switching element), there is a certain risk because of theunfavorable physiological properties of SO₂ in the case of damage to thesensor system and, above all, during the production of the sensorsystems. There is, moreover, a duty to provide identification marking(reference to SO₂).

The object of the present invention therefore consists in finding afiller for sensor systems of temperature-sensitive displacement pickupswhich exhibits virtually the same operational performance but does nothave the disadvantages indicated.

The object is achieved by using 2H-heptafluoropropane (HFC 227).

HFC 227 exhibits virtually the same operational performance as SO₂, islargely inert chemically, and in the temperature range (approximately-30° to +100° C.) of interest has a very good material compatibilitywith all metallic materials normally used for sensor systems.

We claim:
 1. Use of 2H-heptafluoropropane as filler for sensor systems of temperature-sensitive displacement pickups. 